Martín López is an Argentine landlord, but in recent years he felt more like a nervous fugitive. Now based in Madrid, he spent much of 2022 and 2023 mired in anxiety and paperwork—not because he did anything immoral, but because Argentina's rental laws made being a landlord a liability. "Martín López" is an alias. Until late 2023, he rented out his two-bedroom apartment in Buenos Aires' upscale Belgrano neighborhood through a tangle of short-term contracts, never fully sure whether his actions were legal. Argentina's 2020 rent control law, repealed by President Javier Milei in December 2023, had loaded aboveground landlords...